As soon as it was noticed that the propeller pitch indicator did not correspond to the manoeuvring order, the engine-room personnel were immediately advised to take charge of the controllable-pitch operation. The prompt reaction of the crew helped prevent more serious consequences. Inspection of the system in dry-dock revealed that certain parts, including the seals in the propeller hub, were dry and worn. The components of the mechanism inside the shaft that controls the propeller pitch evidently had not been replaced since the vessel was put in service in 1974. The controllable-pitch propeller of the IMPERIAL ST.CLAIR is a category S2 model (Kamewa). A unique feature of this model is that it operates at high pressure and the seals inside the propeller hub are made of rubber instead of steel as on other models. Although Kamewa did not provide the TSB with servicing information on these seals, the recommended replacement interval is not mentioned in Kamewa servicing information obtained from another source. In practice, these components are apparently inspected every 10 years. However, it was not determined why the seals had not been replaced in 22 years or were expected to last that length of time. Seal effectiveness can be verified without dismantling the propeller shaft. However, that procedure does not indicate the wear or flexibility of the seals.Analysis As soon as it was noticed that the propeller pitch indicator did not correspond to the manoeuvring order, the engine-room personnel were immediately advised to take charge of the controllable-pitch operation. The prompt reaction of the crew helped prevent more serious consequences. Inspection of the system in dry-dock revealed that certain parts, including the seals in the propeller hub, were dry and worn. The components of the mechanism inside the shaft that controls the propeller pitch evidently had not been replaced since the vessel was put in service in 1974. The controllable-pitch propeller of the IMPERIAL ST.CLAIR is a category S2 model (Kamewa). A unique feature of this model is that it operates at high pressure and the seals inside the propeller hub are made of rubber instead of steel as on other models. Although Kamewa did not provide the TSB with servicing information on these seals, the recommended replacement interval is not mentioned in Kamewa servicing information obtained from another source. In practice, these components are apparently inspected every 10 years. However, it was not determined why the seals had not been replaced in 22 years or were expected to last that length of time. Seal effectiveness can be verified without dismantling the propeller shaft. However, that procedure does not indicate the wear or flexibility of the seals. The propeller pitch began to show signs of malfunctioning at the Snell Lock. The yaw to port was caused by the propeller pitch falling to zero and was aggravated by the application of astern power. After the first two incidents, checks and tests failed to reveal the cause of the problem. It was decided to dry-dock the vessel for a propeller hub inspection. The loss of propeller pitch was caused by wear of the components in the mechanism of the Kamewa system. The oil seal rings and some components of the propeller pitch control mechanism had not been replaced since the vessel was put in service in 1974. The prompt reaction by the crew and the implementation of strict directives helped to limit damages.Findings The propeller pitch began to show signs of malfunctioning at the Snell Lock. The yaw to port was caused by the propeller pitch falling to zero and was aggravated by the application of astern power. After the first two incidents, checks and tests failed to reveal the cause of the problem. It was decided to dry-dock the vessel for a propeller hub inspection. The loss of propeller pitch was caused by wear of the components in the mechanism of the Kamewa system. The oil seal rings and some components of the propeller pitch control mechanism had not been replaced since the vessel was put in service in 1974. The prompt reaction by the crew and the implementation of strict directives helped to limit damages. The oil tanker IMPERIAL ST.CLAIR grounded because of a loss of thrust from the controllable-pitch propeller while the vessel was being operated in the confined waters of the Seaway at full ahead power. The controllable pitch of the propeller fell to zero because of a drop of pressure in the Kamewa system caused by excessive wear of some components of the mechanism.Causes and Contributing Factors The oil tanker IMPERIAL ST.CLAIR grounded because of a loss of thrust from the controllable-pitch propeller while the vessel was being operated in the confined waters of the Seaway at full ahead power. The controllable pitch of the propeller fell to zero because of a drop of pressure in the Kamewa system caused by excessive wear of some components of the mechanism.